Surface shape measuring system

ABSTRACT

A grating ( 3 ) is disposed to face the measurement target surface of a measurement target object ( 11 ). A light source ( 1 ) irradiates the grating ( 3 ) with illumination light. A camera ( 6 ) captures a moire fringe image formed on the grating ( 3 ) by light passing through the grating  3  and reflected by the measurement target surface. A moving means ( 9 ) changes the distance (H) between the grating  3  and the measurement target surface. An analyzing means ( 8 ) performs an analysis process of obtaining 3-D shape information of the measurement target surface from the image picked up by the camera ( 6 ) in at least two cases in which the distance (H) is set to different values, and obtains, on the basis of the 3-D shape information in each case and the distance H, true 3-D shape information from which the measurement error caused by the inclination of the measurement target surface is eliminated.

1. TECHNICAL FIELD

[0001] The present invention relates to a surface shape measuring systemwhich measures the surface shape of a relatively flat object such as acompact disk, magnetooptical disk, or hard disk.

2. BACKGROUND ART

[0002] Storage media capable of high-density storage such as opticaldisks are recently in great use. They are required to have high flatnessin order to achieve a higher storage density. For this purpose, thesurface shape of a storage medium must be checked in a manufacturingprocess. As a method of performing such surface shape measurement, amoire method has been known. The moire method is a method of measuringthe surface shape of an object from the moire fringes (contour lines ofa surface shape) produced by superimposing a grating and a grating imagedeforming in accordance with the shape of the object when light from apoint light source passes through the grating and strikes the object.

[0003] In the moire method using divergent light, however, the contourline interval (a level difference per moire fringe) increases with anincrease in the distance between a grating and an object. This causes anerror unless the ordinal number (degree n) of a given moire fringe fromthe grating surface can be specified. In addition, in the moire method,since only contour lines are displayed, recesses and projections cannotbe discriminated. The measurement precision can be improved by reducingthe pitch of the grating. If, however, the pitch is reduced, thecontrast of the moire fringes decreases. This limits the contour lineinterval to about 10 μm at most.

[0004] In order to solve such a problem, therefore, a scheme based on acombination of a parallel light moire method and a phase shift methodhas been proposed (e.g., Japanese Patent Laid-Open No. 7-332956). Acharacteristic feature of the parallel light moire method is that lightfrom a point light source 21 is converted into parallel light by using alens 22 to always make the contour line interval constant regardless ofthe distance from a grating 23, as shown in FIG. 8. For this reason,there is no need to determine a degree n of a moire fringe, and no errorbased on the contour line interval is caused. When reflected light isused, a contour line interval Δh is obtained by only an incident (exit)angle θ of light and a pitch p of the grating 23 according to thefollowing equation:

Δh=p/(2tan θ)  (1)

[0005] Referring to FIG. 8, reference numeral 24 denotes a condensinglens for condensing reflected light.

[0006] In the conventional moire method using divergent light, an objecthaving a mirror-reflecting surface such as a glass member or siliconwafer cannot be measured because the reflection angle changes inaccordance with the incident angle which changes depending on theposition (an object having a diffused reflecting surface can be measuredbecause the angle seen by an observer becomes a reflection angle). Incontrast to this, according to the parallel light moire method, since anincident angle and reflection angle remain the same regardless of theposition, even a mirror surface object can be measured.

[0007] In the phase shift method, assuming that discrete informationsuch as contour line fringes is a periodic trigonometric function of alight intensity, the information is handled as continuous information,i.e., the phase of the trigonometric function, to recognize a surfaceshape with a precision higher than the number of contour line fringes.The phase shift method is disclosed in, for example, Tomizawa andYoshizawa, “Phase-Shifting Shadow Moire Method”, Proceedings of JSPEFall Meeting (1991), p. 677.

3. DISCLOSURE OF INVENTION

[0008] [Problem to be Solved by the Invention]

[0009] As described above, even a mirror surface object can be measuredby using the parallel light moire method. In practice, however, in usingthe parallel light moire method, however, if a measurement target objectis a mirror surface object, and the measurement target object has aninclined surface, a measurement error is caused, and the surface shapecannot be accurately measured.

[0010] The reason why such a problem arises will be described below withreference to FIG. 9. As shown in FIG. 9, if the surface of the mirrorsurface object inclines with respect to a horizontal plane (a planeparallel to the grating) by ψ, a normal L1 to the object surfaceinclines by ψ with respect to a normal L0 to the horizontal plane.Therefore, the direction of reflected light at the time of incidence ofparallel light on a horizontal plane (to be referred to as inclinedsurface reflected light hereinafter) shifts by 2ψ with respect toreflected light at the time of incidence of parallel light on ahorizontal plane (to be referred to as a horizontal plane reflectedlight hereinafter).

[0011] In this case, a distance a between an incident point on theobject surface and a point on the grating surface which horizontal planereflected light reaches is obtained by

a=H tan θ  (2)

[0012] where H is the distance between the object surface (incidentpoint) and the grating. In addition, a distance a′ between an incidentpoint on the object surface and a point on the grating surface whichreflected light reaches when inclined is obtained by

a′=H tan(θ+2ψ)  (3)

[0013] According to equations (2) and (3), a difference Δa between thedistances a′ and a is given by

Δa=H{tan(θ+2ψ)−tan θ}  (4)

[0014] As compared with a case wherein parallel light is incident on ahorizontal plane, when parallel light is incident on an inclined objectsurface, contour lines shift by some pitches. Therefore, a measurementerror δh can be expressed as

δh=(Δa/p)Δh  (5)

[0015] According to equations (1) and (4), equation (5) can be modifiedas follows:

δh=H×{tan(θ+2ψ)−tan θ}/(2tan θ)  (6)

[0016] As described above, according to the conventional measurementmethod, if a measurement target object is a mirror surface object andhas an inclined surface, the measurement error δh is caused. Note thatthis problem is common to the moire method and the oblique incidentinterference method of using interference fringes produced by obliquelyincident light as contour line fringes.

[0017] The present invention has been made to solve the above problem,and has as its object to provide a surface shape measuring system whichcan accurately measure the surface shape of a mirror surface object.

[0018] [Means of Solution to the Problem]

[0019] A surface shape measuring system according to the presentinvention comprises an optical element for formation of contour linefringes which is disposed to face a measurement target surface of ameasurement target object, a light source which irradiates the opticalelement with illumination light, a camera which captures an image of thecontour line fringes formed on the optical element by light passingthrough the optical element and reflected by the measurement targetsurface, moving means for changing a distance between the opticalelement and the measurement target surface, and analyzing means forperforming an analysis process of obtaining 3-D shape information of themeasurement target surface from an image picked up by the camera in atleast two cases in which the distance is set to different values, andobtaining true 3-D shape information from which a measurement errorcaused by an inclination of the measurement target surface iseliminated, on the basis of the 3-D shape information in each of thecases and the distance.

[0020] In an example of the arrangement of the surface shape measuringsystem according to the present invention, the optical element is agrating, and the contour line fringes are moire fringes formed bysuperimposing the grating and a grating image passing through thegrating and reflected by the measurement target surface.

[0021] In an arrangement example of the surface shape measuring systemaccording to the present invention, the optical element is a prism, andthe contour line fringes are interference fringes formed bysuperimposing light reflected by a prism surface and light passingthrough the prism and reflected by the measurement target surface.

[0022] In an arrangement example of the surface shape measuring systemaccording to the present invention, the analyzing means obtains a linearfunction representing a relationship between the distance and the 3-Dshape information on the basis of the 3-D shape information in at leastthe two cases in which the distance is set to different values, and setsa functional value obtained when the distance is 0 as true 3-D shapeinformation from which the measurement error is eliminated.

4. BRIEF DESCRIPTION OF DRAWINGS

[0023]FIG. 1 is a block diagram showing the arrangement of a surfaceshape measuring system according to an embodiment of the presentinvention;

[0024]FIG. 2 is a flow chart showing the operation of the surface shapemeasuring system in FIG. 1;

[0025]FIG. 3 is a graph showing the relationship between the lightintensity and the distance from a grating;

[0026]FIG. 4 is a graph showing the relationship between the measurementdata and the distance between the grating and the measurement targetsurface;

[0027]FIG. 5 is a graph showing an example of the measurement resultobtained by the surface shape measuring system in FIG. 1;

[0028]FIG. 6 is a view for explaining an oblique incident interferencemethod;

[0029]FIG. 7 is a view for explaining the reason why a measurement erroris caused in the oblique incident interference method;

[0030]FIG. 8 is a view for explaining a conventional parallel lightmoire method; and

[0031]FIG. 9 is a view for explaining the reason why a measurement erroris caused on a mirror surface object.

5. BEST MODE FOR CARRYING OUT THE INVENTION

[0032] An embodiment of the present invention will be described indetail next with reference to the accompanying drawings. FIG. 1 is ablock diagram showing the arrangement of a surface shape measuringsystem according to an embodiment of the present invention. FIG. 2 is aflow chart showing the operation of the surface shape measuring systemin FIG. 1. The surface shape measuring system in FIG. 1 includes a lightsource 1 for outputting monochromatic point light such as a helium neonlaser beam, a lens 2 for converting the illumination light emitted fromthe light source 1 into parallel light, a grating 3 which is an opticalelement disposed to be substantially parallel to the measurement targetsurface of a measurement target object 11 placed on a stage 10, acondensing lens 4 for condensing the moire fringe image formed bysuperimposing the grating 3 and a grating image passing through thegrating 3 and reflected by the measurement target surface of themeasurement target object 11, a slit 5 for capturing only reflectedcomponents from the measurement target object 11 by eliminatingdiffracted components from the grating 3, a camera 6 for capturing amoire fringe image, an A/D converter 7 for converting the image signaloutput from the camera 6 into digital data, an analyzing means 8 forobtaining 3-D shape information of the measurement target surface fromthe moire fringe image, and a moving means 9 such as a piezoelectricactuator or stepping motor which changes the distance between thegrating 3 and the measurement target surface by vertically moving thegrating 3 while keeping it parallel to the measurement target surface.The grating 3 is formed from a glass plate or the like which haslight-shielding portions arranged at a predetermined pitch p.

[0033] The operation of the surface shape measuring system according tothis embodiment will be described below. First of all, the analyzingmeans 8 controls the moving means 9 to move the grating 3 so as to set adistance H between the grating 3 and the measurement target surface ofthe measurement target object 11 to a first predetermined value H1(e.g., 8 mm) (step 101 in FIG. 2).

[0034] Subsequently, the analyzing means 8 captures the moire fringeimage formed on the measurement target object 11 (step 102). Theillumination light emitted from the light source 1 is converted intoparallel light by the lens 2. This parallel light passes through thegrating 3 and strikes the measurement target object 11 placed on thestage 10 to form the shadow of the grating 3 on the measurement targetobject 11. As a consequence, the image of the grating 3 which has passedthrough the grating 3, struck on the object 11, and deformed inaccordance with the surface shape of the object 11 is superimposed onthe grating 3 to form moire fringes in the form of contour linescorresponding to the surface shape of the object 11.

[0035] The moire fringe image is condensed by the condensing lens 4,passes through the slit 5, and strikes the camera 6. The camera 6converts the incident light into an electrical signal. In this manner,the moire fringe image signal is output from the camera 6. This imagesignal is converted into digital data by the A/D converter 7. Theanalyzing means 8 captures the image data output from the A/D converter7 and stores the image data in an internal memory. With this process,image capturing is completed.

[0036] After the image capturing, the analyzing means 8 checks whetherimage capturing has been done four times (step 103). In this case, sinceimage capturing has not been done four times, the analyzing means 8controls the moving means 9 to move the grating 3 upward by Δh/4 (step104), and performs image capturing in step 102 again.

[0037] The processing in steps 102 to 104 is repeated until imagecapturing is done four times. As a consequence, the image data obtainedwhen the distance between the grating 3 and the measurement targetobject 11 is H1, H1+Δh/4, H1+Δh/2, and H1+3Δh/4 are stored in the memoryof the analyzing means 8. Note that an interval Δh between contour linefringes can be calculated by equation (1).

[0038] The analyzing means 8 then controls the moving means 9 to movethe grating 3 so as to set the distance between the grating 3 and themeasurement target surface of the measurement target object 11 to asecond predetermined value H2 (e.g., 16 mm) when it is assumed that themeasurement target surface is perfectly flat (step 105). The processingin steps 106 to 108 is the same as that in steps 102 to 104. In additionto the above image data corresponding to four image capturingoperations, the image data obtained when the distance between thegrating 3 and the measurement target object 11 is H2, H2+Δh/4, H2+Δh/2,and H2+3Δh/4 are stored in the memory of the analyzing means 8.

[0039] Subsequently, the analyzing means 8 performs the first analysisprocess of calculating 3-D shape information of the measurement targetsurface of the measurement target object 11 (a relative distance h(X, Y)between the grating 3 and a point with coordinates (X, Y) on themeasurement target surface) by using the phase shift method for theimage data corresponding to four image capturing operations which arecaptured in the processing in steps 101 to 104 (step 109).

[0040] A light intensity I(X, Y) of a moire contour line fringe atcoordinates (X, Y) on the measurement target surface of the measurementtarget object 11 is a periodic function of the relative distance h(X, Y)with respect to the grating 3. Assuming that this periodic function is asine wave, the light intensity is expressed as shown in FIG. 3 and canbe represented as

I(X, Y)=a(X, Y)+b(X, Y)×cos(2πh(X, Y)/Δh+φ)  (7)

[0041] where φ is the phase, a(X, Y) is the light intensity offset, andb(X, Y) is the amplitude of light intensity. These values changedepending on light source intensity irregularity, a flaw on the lens,the pattern attached to the measurement target object 11, and thereflectance of the measurement target object 11.

[0042] In order to obtain the relative distance h(X, Y) between themeasurement target object 11 and the grating 3, the unknowns a(X, Y) andb(X, Y) of equation (7) must be eliminated. For this purpose, the phaseφ is changed to 0, π/2, π, and 3π/2, and light intensities I0(X, Y),I1(X, Y), I2(X, Y), and I3(X, Y) in the respective cases are obtained.These light intensities in these four cases can be expressed as:$\begin{matrix}\begin{matrix}{{I\quad 0\left( {X,Y} \right)} = {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{{\cos \left( {2\pi \quad {{h\left( {X,Y} \right)}/\Delta}\quad h} \right)}}\end{matrix} & (8) \\\begin{matrix}{{I\quad 1\left( {X,Y} \right)} = {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{{\cos \left( {{2\pi \quad {{h\left( {X,Y} \right)}/\Delta}\quad h} + {\pi/2}} \right)}} \\{{= {{a\left( {X,Y} \right)} - {{b\left( {X,Y} \right)} \times}}}} \\{{\sin \left( {2\pi \quad {{h\left( {X,Y} \right)}/\Delta}\quad h} \right)}}\end{matrix} & (9) \\\begin{matrix}{{I\quad 2\left( {X,Y} \right)} = {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{{\cos \left( {{2\pi \quad {{h\left( {X,Y} \right)}/\Delta}\quad h} + \pi} \right)}} \\{{= {{a\left( {X,Y} \right)} - {{b\left( {X,Y} \right)} \times}}}} \\{{\cos \left( {2\pi \quad {{h\left( {X,Y} \right)}/\Delta}\quad h} \right)}}\end{matrix} & (10) \\\begin{matrix}{{I\quad 3\left( {X,Y} \right)} = {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}} \\{{\cos \left( {{2\pi \quad {{h\left( {X,Y} \right)}/\Delta}\quad h} + {3{\pi/2}}} \right)}} \\{{= {{a\left( {X,Y} \right)} + {{b\left( {X,Y} \right)} \times}}}} \\{{\sin \left( {2\pi \quad {{h\left( {X,Y} \right)}/\Delta}\quad h} \right)}}\end{matrix} & (11)\end{matrix}$

[0043] The following equation can be obtained by eliminating a(X, Y) bysubtracting equation (10) from equation (8):

I0(X, Y)−I2(X, y)=2b(X, Y)cos(2πh(X, Y)/Δh)  (12)

[0044] The following equation can be obtained by eliminating a(X, Y) bysubtracting equation (9) from equation (11):

I3(X, Y)−I1(X, Y)=2b(X, Y)sin(2πh(X, Y)/Δh)  (13)

[0045] According to equations (12) and (13), the relative distance h(X,Y) can be obtained by

h(X, Y)=(Δh/2π)tan⁻¹{(I3(X, Y)−I1(X, Y))/(I0(X, Y)−I2(X, Y))}  (14)

[0046] According to equation (14), the relative distance h(X, Y) betweenthe grating 3 and the point with the coordinates (X, Y) on themeasurement target surface can be obtained without being influenced bydifferences in the offset a(X, Y) and amplitude b(X, Y). In order toshift the phase of the moire fringes π/2 at a time, the distance betweenthe measurement target object 11 and the grating 3 is moved Δh/4 at atime, image data corresponding to four image capturing operations iscaptured, the light intensity I(X, Y) of each image data is obtained foreach set of coordinates X and Y, and the relative distance h(X, Y) isobtained by equation (14).

[0047] In this case, the light intensity obtained from the image dataobtained when the distance between the grating 3 and the measurementtarget object 11 is H1 is the light intensity I0(X, Y) when the phase φis 0. The light intensities obtained from the image data correspondingto the distances H1+Δh/4, H1+Δh/2, and H1+3Δh/4 are the lightintensities I1(X, Y), I2(X, Y), and I3(X, Y) when the phase φ is π/2, π,and 3π/2. Therefore, the relative distance h(X, Y) can be obtained fromthe image data which correspond to four image capturing operations andare captured in the processing in steps 101 to 104.

[0048] The analyzing means 8 then performs the second analysis processof calculating the 3-D shape information of the measurement targetsurface of the measurement target object 11 by using the phase shiftmethod for the image data which correspond to four image capturingoperations and are captured in the processing in steps 105 to 108 (step110). The second analysis process can be done in the same manner as thefirst analysis process.

[0049] That is, the light intensities obtained from the image dataobtained when the distance between the grating 3 and the measurementtarget object 11 is H2, H2+Δh/4, H2+Δh/2, and H2+3Δh/4 are the lightintensities I0(X, Y), I1(X, Y), I2(X, Y), and I3(X, Y) when the phase φis 0, π/2, π, and 3π/2. The relative distance h(X, Y) can therefore beobtained from the image data which correspond to four image capturingoperations and are captured in the processing in steps 105 to 108 byusing equation (14).

[0050] The analyzing means 8 then eliminates a measurement error δhcaused by the inclination of the measurement target surface from therelative distances h(X, Y) calculated in steps 109 and 110 (step 111).As indicated by equation (6), the measurement error δh is proportionalto the distance H between the grating 3 and the measurement targetobject 11. Therefore, letting h1(X, Y) be the distance calculated instep 109 from the image data corresponding to four image capturingoperations in steps 101 to 104, and h2(X, Y) be the distance calculatedin step 110 from the image data corresponding to four image capturingoperations in steps 105 to 108, a true measurement value h0(X, Y) afterthe elimination of the measurement error δh can be given by

h0(X, Y)=h1(X, Y)−h1(X, Y)×(h1(X, Y)−h2(X, Y))/(H1−H2)  (15)

[0051]FIG. 4 shows the relationship represented by equation (15). Themeasurement value h0(X, Y) is the value when the distance H between thegrating 3 and the measurement target object 11 is 0. As a condition inwhich equation (15) holds, it is required that the distances H1 and H2be sufficiently large with respect to the maximum value (e.g., about 10to 100 μm) of the level differences on the measurement target surface ofthe measurement target object 11.

[0052] The distances H1 and H2 are measured by a mechanical detectormounted on the moving means 9, and hence have measurement errors. Forthis reason, H2 is set to be about two to three times H1, so thedifference between H1 and H2 becomes sufficiently large with respect tothe maximum value of the level differences. Another condition for thedetermination of the distances H1 and H2 is that the contrast of moirefringes is clear. In consideration of the above conditions, in thisembodiment, H1 is set to 8 mm, and H2 is set to 16 mm.

[0053] In this manner, the true height h0(X, Y) of the object surfaceafter the elimination of the measurement error δh can be calculated byequation (15).

[0054]FIG. 5 shows an example of the measurement result obtained by thesurface shape measuring system according to this embodiment. FIG. 5shows the result obtained by measuring the shape of the concave surfaceof a concave mirror as the measurement target object 11, which has aradius of 30,000 mm. As is obvious, although the measurement dataobtained when the distance H is 8 mm and 16 mm greatly deviate from theideal curve, the data after error correction in step 111 is broughtclose to the ideal curve.

[0055] In this embodiment, error correction according to the presentinvention is applied to a combination of the parallel light moire methodand the phase shift method. However, the present invention may beapplied to the oblique incident interference method. As shown in FIG. 6,in the oblique incident interference method, a prism 12 is disposed asan optical element to face the measurement target surface of themeasurement target object 11. The prism 12 is irradiated withillumination light. The light reflected by the prism surface is thensuperimposed on the light which passes through the prism 12, isreflected by the measurement target surface, and strikes the prism 12again, thereby forming interference fringes on a screen 13. Theseinterference fringes are picked up by a camera 14 to obtain 3-D shapeinformation on the basis of the interference fringe image.

[0056] In this oblique incident interference method, the phasedifference obtained when the measurement target surface of themeasurement target object 11 inclines from a horizontal plane by ψbecomes the measurement error δh. This measurement error δh is expressedas

δh=(2π/λ){(AC−AB)}−nDC)  (16)

[0057] where λ is the wavelength of incident light, n is the degree, andAC, AB, and DC are the distance between a point A and a point C, thedistance between the point A and a point B, and the distance between apoint D and the point C in FIG. 7. The distances AC, AB, and DC can beobtained as follows:

AC={1/cos(θ′+ψ)}H  (17)

AB={1/cos θ′)H  (18)

DC=BC sin(90−θ)  (19)

[0058] According to equation (19), the distance BC between the point Band the point C is obtained by

BC=H{tan(θ+2ψ)−tan θ}  (20)

[0059] As described above, since the measurement error δh in the obliqueincident interference method is proportional to the distance H betweenthe prism 12 and the measurement target surface (point A) of themeasurement target object 11, the measurement error can be eliminated byapplying the present invention.

[0060] In addition, in this embodiment, the 3-D shape information h1(X,Y) and 3-D shape information h2(X, Y) are obtained when the distancebetween the grating 3 and the measurement target object 11 is H1 and H2,and the true 3-D shape information h0(X, Y) is obtained from thesepieces of information. However, the distance H may be set to three ormore values. In this case, 3-D shape information may be obtained foreach distance H in the same manner as in the first embodiment, a linearfunction (a function representing the straight line in FIG. 4)representing the relationship between the distance H and the 3-D shapeinformation may be obtained from these pieces of 3-D shape informationby using the least squares method, and a functional value when thedistance H is 0 may be obtained as the true 3-D shape information h0(X,Y) from this function.

[0061] In addition, in this embodiment, the measurement target surfaceof the measurement target object 11 is substantially parallel to thegrating 3. In practice, however, the grating 3 has a slight inclinationwith respect to the measurement target surface (for example, a height ofabout 100 μm with respect to the measurement target object 11 100 mmsquare). This is because it prevents reflected/diffracted light from thegrating 3 from striking the camera 6. Reflected/diffracted light fromthe grating 3 is offset from the optical path of reflected/diffractedlight from the measurement target surface of the measurement targetobject 11 and shielded by the slit 5. This light is not thereforeprojected on the camera 6. Note that since the inclination of thegrating 3 is small, it has no influence on the calculation in step 111.

[0062] In this embodiment, a helium neon laser as a monochromatic lightsource having a short coherent length is used as the light source 1.However, the present invention is not limited to this. A combination ofsodium lamp or mercury lamp as an incoherent light source and a filterthat transmits only a specific emission spectrum may be used.

[0063] As has been described above, according to this embodiment, ananalysis process of obtaining the 3-D shape information of a measurementtarget surface from the image picked up by the camera 6 is performed inat least two cases in which the distance between an optical element andthe measurement target surface is set to different values, and acomputation is performed on the basis of the 3-D shape informationobtained in each case and the distance, thereby obtaining true 3-D shapeinformation from which the measurement error caused by the inclinationof the measurement target surface is eliminated. As a consequence, thesurface shape of a mirror surface object can be accurately measured.

6. INDUSTRIAL APPLICABILITY

[0064] As described above, the present invention is suitable for themeasurement of the surface shape of a mirror surface object.

1. A surface shape measuring system characterized by comprising: anoptical element for formation of contour line fringes which is disposedto face a measurement target surface of a measurement target object; alight source which irradiates said optical element with illuminationlight; a camera which captures an image of the contour line fringesformed on said optical element by light passing through said opticalelement and reflected by the measurement target surface; moving meansfor changing a distance between said optical element and the measurementtarget surface; and analyzing means for performing an analysis processof obtaining 3-D shape information of the measurement target surfacefrom an image picked up by said camera in at least two cases in whichthe distance is set to different values, and obtaining true 3-D shapeinformation from which a measurement error caused by an inclination ofthe measurement target surface is eliminated, on the basis of the 3-Dshape information in each of the cases and the distance.
 2. A surfaceshape measuring system according to claim 1, characterized in that saidoptical element is a grating, and the contour line fringes are moirefringes formed by superimposing said grating and a grating image passingthrough said grating and reflected by the measurement target surface. 3.A surface shape measuring system according to claim 1, characterized inthat said optical element is a prism, and the contour line fringes areinterference fringes formed by superimposing light passing through saidprism and reflected by the measurement target surface and lightreflected by a prism surface.
 4. A surface shape measuring systemaccording to claim 1, characterized in that said analyzing means obtainsa linear function representing a relationship between the distance andthe 3-D shape information on the basis of the 3-D shape information inat least the two cases in which the distance is set to different values,and sets a functional value obtained when the distance is 0 as true 3-Dshape information from which the measurement error is eliminated.